Field of the Invention
The present invention relates to an image forming apparatus which forms an image on recording material using an electrophotographic system.
Description of the Related Art
Examples of conventional image forming apparatuses using an electrophotographic system include an electrophotographic copier, an electrophotographic printer (such as an LED printer and a laser beam printer), and an electrophotographic facsimile device. In an image forming apparatus of this type, a surface of an electrophotographic photosensitive member (hereinafter, referred to as a photosensitive drum or a drum) is uniformly charged by a primary charger and the charged photosensitive drum surface is exposed by an exposing apparatus to form an electrostatic latent image. The electrostatic latent image is then developed by a developing apparatus to form a developer image (hereinafter, referred to as a toner image) and the toner image is transferred to recording material such as a sheet by a transferring apparatus. Subsequently, the toner image is output by fixing the toner image onto the recording material as a fixed image by a fixing apparatus. Toner remaining on the surface of the photosensitive drum after toner image transfer is cleaned by a cleaning apparatus and the photosensitive drum stands by for a next image forming operation.
In recent years, an increasing number of image forming apparatuses are mounted with charging apparatuses adopting a contact charging system which have become a mainstream in charging apparatuses. Most contact charging systems utilize roller charging in which a conductive roller is used as a contact charging member and voltage is applied by bringing the conductive roller into contact with a photosensitive drum. There are further a DC system in which a surface of a photosensitive drum is charged by only applying DC voltage to a contact charging member and an AC superposition system in which a surface of a photosensitive drum is charged by superimposing DC voltage on AC voltage and applying the superimposed voltage to a contact charging member. According to the AC superposition system, while there is an advantage in that the surface of the photosensitive drum can be uniformly charged, discharges occur repetitively in accordance with a frequency of the AC voltage. As a result, the surface of the photosensitive drum sustains damage, an amount of abrasion increases, and product life of the photosensitive drum is reduced. In contrast, with the DC system, since the number of discharges that occur in minute gaps is smaller than in an AC superposition system, less damage is sustained by the photosensitive drum and a longer product life of the photosensitive drum is achieved. However, charging a photosensitive drum particularly using a DC system creates the following problems.
On a surface of a photosensitive drum after image formation, surface potential has become nonuniform in accordance with a formed image. When charging is next performed in this state, depending on the previous formed image, uniform charging cannot be realized. As a result, surface potential of the photosensitive drum when exposed by an exposing apparatus such as a laser may also become nonuniform. In other words, a so-called ghost image may occur. More specifically, when a halftone image is formed after forming a pattern with high contrast, a so-called ghost image occurs in which the previous image pattern emerges in the halftone.
In consideration thereof, Japanese Patent Application Laid-open No. 2009-42738 proposes a configuration in which a surface of a photosensitive drum prior to charging is uniformly irradiated by a pre-charge exposing apparatus having a light source such as an LED. Accordingly, uniform charging is realized in a next charging operation by averaging potential of image portions (potential of bright parts) and potential of non-image portions (potential of dark parts) of the surface of the photosensitive drum when exposed by the exposing apparatus to prevent the occurrence of a ghost image.
However, in recent years, there have been demands for further extending product life of image forming apparatuses having a photosensitive drum for the purpose of reducing running cost. While the DC system is advantageous in that the amount of abrasion of the surface of a photosensitive drum is small, the surface of the photosensitive drum nevertheless deteriorates due to discharges and is subject to scuffing due to passing of paper or a cleaning member coming into contact with the surface. Consequently, a film thickness of the photosensitive drum decreases. In addition, while suppressing ghost images, since pre-charge exposure also lowers surface potential of the photosensitive drum prior to charge, an amount of discharge increases and, as a result, an amount of abrasion of the photosensitive drum increases.
In order to solve this problem, conceivably, an initial film thickness of a photosensitive drum may be increased while performing irradiation for preliminary exposure immediately prior to image formation (immediately prior to exposure) in order to minimize irradiation time of pre-charge exposure. However, Japanese Patent Application Laid-open No. 2009-42738 describes that an overcharge (abnormal discharge) of charged potential sometimes occurs when charging a region after irradiation for preliminary exposure to create a mesh-like abnormally discharged image. This occurs prominently when an initial film thickness of a drum is increased from the perspective of extending product life.